Fix code quality: trailing whitespace, unused imports, clippy warnings

- Fix trailing whitespace in kex.rs and s3.rs
- Add missing KexProposal import in kex_complete.rs
- Auto-fix clippy warnings across all crates
- All 153 tests pass

markbase-core/src/cli/tools/mod.rs

@@ -17,4 +17,4 @@ pub async fn handle_tools_command(cmd: ToolsCommands) -> anyhow::Result<()> {
         ToolsCommands::Test(c) => test::handle_test_command(c)?,
     }
     Ok(())
-}
+}